Many attempts have been made in the past to provide reliable systems for maintaining an optimum of moisture in the soil for growing crops such as lawns, flowers, gardens and farm produce. Automatic systems are well known which operate on a predetermined time sequence which may be adjusted for seasonal and atmospheric conditions. Some systems utilize moisture sensing probes inserted into the soil for sensing the moisture content. However, a number of drawbacks have been encountered in known buried sensing probes, one of them being insufficient sensitivity, especially at high saturation levels. For growing turf, it is commonly desired to have soil moisture at between 80 and 100% of saturation and that is where moisture probe sensitivity should ideally be optimum. Another problem with prior art moisture sensing probes is that they tend to have a relatively short useful life, being subject to corrosion.
Various configurations of moisture sensing probes have been employed. Some of these are configured as a pair of probes spaced apart in the soil where the system measures the conductivity between the probes. Another moisture sensor, as shown in U.S. Pat. No. 4,652,811, employs a group of resistors, each having a point contact, and a distributed electrode, the point contacts and the electrode being in contact with the soil being monitored. The transfer characteristic and thereby the sensitivity of this device is substantially the same whether the soil is dry or saturated.
Certain characteristics of the porous medium and the moisture therein give rise to some of the difficulties in measuring the moisture content. Damp mediums, such as soils, are electrolytic and are characterized mainly by a number of electrical parameters such as; electrical conductivity, electrical capacitance, and electrical inductance. These parameters vary with the characteristics of the medium. The main characteristics which influence these parameters are the medium's density, structural form, chemical composition and moisture content.